In recent years, with the development of portable rechargeable electrical devices such as an electrical shaver, a nickel metal hydride (Ni-MH) rechargeable battery is becoming to be widely used as a rechargeable battery. With the widespread use of the nickel metal hydride rechargeable battery, a charging function of a rechargeable electrical device is becoming more advanced. There has been a remarkable increase in the number of rechargeable electrical devices which charge their rechargeable batteries rapidly with a charging current larger than the conventional charging current so as to enable to finish charging their rechargeable batteries in a shorter charging time than the conventional rechargeable electrical device. Even after the rapid charge, the rechargeable electrical device using the nickel metal hydride rechargeable battery as a rechargeable battery continues charging with a minute electric current having a constant current value smaller than the charging current value in the rapid charge in order to either compensate for self-discharge of the rechargeable battery or activate the rechargeable battery in an inactive state. The above-described charging with a minute electric current having a constant current value is called a trickle charge.
By the way, in rechargeable electrical devices comprising a rechargeable battery, it is generally necessary to prevent overcharging of the rechargeable battery so as to prevent overheating and explosion of the rechargeable battery. Thus, conventionally, a microcomputer in many such rechargeable electrical devices is provided with a function to detect a residual capacity of the rechargeable battery (residual capacity detection function) so as to stop (end) charging the rechargeable battery when the microcomputer detects that the residual capacity has become 100% (full charge). However, in the full charge detection method using the residual capacity detection function, it is required to keep the microcomputer active to control at least the amount of charged current to the rechargeable battery and the amount of discharged current from the rechargeable battery to the load, resulting in the problem of power consumption in the microcomputer.
On the other hand, a nickel metal hydride rechargeable battery as a rechargeable battery has a characteristic that in being charged, it reaches a state of nearly full charge when the value of the battery voltage has become slightly lower than the peak value after the value of the battery voltage passes the peak value. Thus, there has been an increase in the number of rechargeable electrical devices, comprising a nickel metal hydride rechargeable battery, each of whose microcomputers determines that the battery has reached a state of full charge when the value of the battery voltage has become slightly lower than the peak value after passing the peak value, so as to stop the rapid charge (refer to Patent Document 1), using the above-described characteristic. Such rechargeable electrical devices need not keep the microcomputer activate to control the amount of charged current and the amount of discharged current, making it possible to reduce power consumption, in contrast to the above-described conventional rechargeable electrical device with the above-described residual capacity detection function.
However, the above-described rechargeable electrical devices, which stop the rapid charge when the value of the battery voltage has become slightly lower than the peak value after passing the peak value, cannot make an inactive nickel metal hydride rechargeable battery (hereafter referred to as an inactive battery) full charge only with the rapid charge.
Referring to FIG. 8, the above-described point will be described. As described above, when the rapid charge on an active battery, which is not in an inactive state, is continued, the active battery reaches a state of nearly full charge when the value of the battery voltage has become slightly lower than the peak value (for example, after 60 minutes from the start of the rapid charge shown in FIG. 8) after the value of the battery voltage passes the peak value. However, in general, when the rapid charge on the inactive battery has been done, the value of the battery voltage often reaches a peak value, which is different from the above-described peak value corresponding to the full charge, within a certain time period from starting the charge. For this reason, when the value of the battery voltage has passed the peak value different from the above-described primary peak value (for example, after 5 minutes from the start of the rapid charge shown in FIG. 8), such conventional rechargeable electrical devices often misdetermine that the rechargeable battery has reached a state of full charge, thereby stopping the rapid charge before full charge. As a countermeasure against this insufficient charge occurred in the inactive battery, such conventional rechargeable electrical devices continues the trickle charge after the stop of the rapid charge so as to simultaneously perform activating the inactive battery and charging the rechargeable battery to full charge by compensating for the capacity, shown by the two-headed arrow in FIG. 8, which could not be filled by the rapid charge.